In the production of coated webs, such as photographic films and papers, it is necessary to perform measurements as part of the production process to ascertain that the coatings are uniform along the length of the web. It is well known to use what is generally called a neutral density flasher for this purpose. A neutral density flasher is apparatus that is used to expose an elongated length of the film to a constant intensity, white light source so that later examination with a densitometer, after processing of the film, can determine variations such as streaks, spots and repetitive anomalies in the film. A particular example when detection of repetitive anomalies is particularly important is in the case of movie film for which repetitive variations in film density are of concern since these defects can cause flickering, a phenomenon which is readily observable to the human eye as the film is played.
Existing neutral density flashers are illustrated schematically in FIG. 1 as including a light exposure box 10 positioned between film supply and takeup reels 12 and 14. As an unexposed strip of film 16 passes over idler/tensioning rollers 17 through the exposure box at some fixed speed, it is exposed to radiation such as a constant intensity white light source 18 to uniformly expose an elongated length of the film, typically upwards of 100'. After subsequent processing (developing) of the film, it is then passed through a densitometer to measure the film density along its exposed length to determine the existence of any undesired variations in the film density characteristic that might be cause for rejecting the film batch from which the test strip was taken. In the case of photographic paper, reflectance rather than density would be the measured characteristic.
Flashers of this type have the problem that repetitive variations in density or reflectance of the film or paper are difficult to isolate. This is because the flasher itself can induce its own variations during the exposure process. For example, reel drive speed variations and motor and roller vibrations can disrupt the constant speed of the film passing through the exposure box causing film jitter which, in turn, adversely affects the total integrated exposure of the film strip over successive increments of the film. Attempts to minimize the effects of frequency variations include using lower light source intensities and longer time exposure to smooth out the fluctuations. To reduce the effect of motor and roller vibrations, high grade bearings and highly stable motor controls are used which significantly increases the complexity and cost of such flasher systems.
It is therefore an object of the invention to provide a web inspection system and a corresponding inspection method that is simpler and more cost effective than systems and methods using flashers of the type described above.
It is a further object of the invention to provide a web inspection system and method utilizing web flasher apparatus that give selected web characteristic measurements, either density or reflectance, that are independent of system induced anomalies such as drive speed variations and film jitter caused by motor and roller vibrations.